Power transmissions are complicated machines, packing many mechanical devices into ever-smaller packages in order to meet cost and weight goals. A present-day transmission may use hydraulic bands to change gearing ratios and thus speeds. A simple two-speed transmission, such as one depicted in U.S. Pat. No. 5,588,928, is used to describe the involved gear and friction elements, and their functions during gear changes. FIG. 1 depicts a transmission consisting of a simple planetary gear unit 1 having an annulus gear 2 coupled with input shaft 3, a sun gear 4 connected with brake drum 5, and a planet carrier 6 connected with output shaft 7. Planet gears 8 mesh with annulus gear 2 and sun gear 4. A self-synchronizing friction band 10 is engaged to hold the drum 5 and the sun gear 4 attached thereto stationary to set the transmission in low gear. The transmission is upshifted to direct drive by applying multi-plate clutch 9 and by disengaging the friction band 10 to lock the planetary gear set for unitary rotation.
In FIG. 2, the friction band 10 encircling the drum 5 has friction lining 11 attached to its inner surface. The band 10 also has lugs 12, 13 secured to each end of the band; one lug 12 to the apply end and another lug 13 to the reaction end. Typically, the friction band actuating system 14 is housed inside a servo chamber 15 extending transversely in a transmission case 16. The main components in the system are the apply piston 17 and the reaction piston 18. Both pistons are subjected to the same pressure regulated by an exhaust control valve 19, which is attached to the reaction piston guide rod 20, responding to the axial movement of reaction piston 18. Chamber 15 is enclosed by a servo cover 22, which includes cylindrical surfaces and oil passages for both pistons as well as an elastomer ring 24 for sealing purposes. A complicated system to apply and release hydraulic pressure causes the band or bands to contract or relax, thus engaging or releasing a drive shaft encircled by the bands. Control system 25 for the selfsynchronized friction band includes a shift valve 26 and a mode valve 27, including ball 28 and spring 29. Ball 30 with seat 23 forms another valve. Hydraulic fluid or oil is supplied and directed through a series of pistons, accumulators, and chambers to control the bands.
Such complicated devices as this brake-band actuated transmission tend to have many components that must interact in a prescribed manner for correct operation. These parts and the resulting transmission are costly. The transmissions are subject to oil leaks. Wear may occur in many parts of the transmission, including the valve seats, the pistons, and the bands themselves. What is needed is a power transmitter having fewer parts and operating in a simpler fashion to add speed ranges to a mechanical transmission. Also, what is needed is a power transmitter that will shift and transmit power with fewer components and less cost, and in which the components are capable of acting simply and reliably to deliver mechanical power.